The invention relates to soldering and fluxes used in soldering processes. More particularly, the invention relates to a soldering process using a liquid flux.
Soldering is one of the oldest methods of joining two or more metal articles. Soldering involves the joining of two metallic surfaces by flowing between them by capillary attraction alloy or solder metals of lower melting point than the metals to be joined. The metal surfaces remain unmelted, but are solidly united when the solder alloy or solder metal solidifies.
Preparation of the joint surfaces is an important factor in soldering. With few exceptions, a flux composition is utilized. Fluxes are used to prevent the oxidation of the filler metal and of the surfaces of the metal being joined during the heating. The flux will also dissolve oxides which naturally exist on most metal surfaces as well as those that may form during the heating operation. Additionally, fluxes influence the surface-tension equilibrium in the direction of solder spreading.
Fluxes are applied to metal surfaces to be soldered by many methods, for example, brushing, spraying, dipping, etc. Generally, a liquid vehicle is utilized in the application of the flux, e.g., a petroleum solvent or water. The use of a liquid vehicle has certain advantages and disadvantages. One advantage is ease of application. One disadvantage occurs, however, in assemblies where the parts to be soldered are pretinned, assembled, dipped in a flux-containing bath, and heated to soldering temperature in an oven. The solvent or liquid vehicle must be evaporated first before soldering occurs and this results in a large loss of energy. If the solvent is non-aqueous, it also creates a large air pollution problem, due to emissions of volatile constituents into the atmosphere. Thus, it is desirable to use a flux which does not require a liquid vehicle for application.
Phosphoric acid is a well-known liquid flux, see for example, U.S. Pat. No. 3,985,587. Phosphoric acid, although being an excellent flux, creates many additional problems, particularly with automated soldering processes as are utilized in the automotive and electronics industries. One problem with phosphoric acid is that it is highly corrosive. Special safety precautions must be taken to protect workers using phosphoric acid. Secondly, special corrosion resistant machinery must be utilized. Thirdly, the phosphoric acid residues which remain after the soldering is accomplished must be removed since they can corrode and weaken the solder joint. Also, the excess flux must be removed since it can cause corrosion of the pieces soldered. Thus, many soldering processes also include a costly washing step to clean pieces of excess flux.
Another desirable feature of fluxes is that of lubricity. In automatic soldering processes, there is much moving machinery and lots of metal-to-metal contact as the parts to be soldered are arranged in orderly arrays within metal jigs after contact with the fluxing liquid. Thus, it is desirable that the flux act as a lubricant to the continuous flow of fixed parts as they are collected and assembled by automatic stacking machines.
Solid wax fluxes containing the phosphorus-containing acids of the present invention are disclosed in our copending application Ser. No. 811,505, filed June 30, 1977.